Abstract 4510: Linking growth factors and immune actors: PTEN/PI3K signaling in cancer and the response to cytokines

Background: The PTEN/PIK3 pathway plays a critical role in the cellular response to growth factors. Furthermore, aberrant activation of PI3K signaling is a known hallmark of many forms of cancer. While the role of PI3K signaling in the context of growth factor biology has been well studied, less is known about its activity in the cellular response to cytokines such as interferons (IFNs). We examined how PI3K activation by PTEN inhibition would affect interferon signaling pathways using an in vitro experimental model system. Methods: Cultured human HCT 116 colorectal carcinoma cells and isogenic PTEN (-/-) cells were treated with escalating doses of IFNγ, IFNα2β, and IFNλ. The response to IFNs was evaluated by measuring mRNA levels via RT-PCR and cell surface expression by flow cytometry of MHC Class I and Class II at sequential timepoints. Western blotting analysis was performed to measure levels of total AKT, phospho-AKT (pAKT), PTEN, STAT1, and pSTAT1. Additional pharmacologic studies were performed using cultured HPV-negative SqCC/Y1 oSCC cells treated with IFN, the PTEN inhibitor VO-OHpic, and AKT activator SC79. Further experiments were also done in an isogenic PTEN knockdown BRAFV600E mutant a375 melanoma cell line. Clinical correlates utilizing immunohistochemistry (IHC) on human head and neck cancers were conducted. Results: IFN-γ- induced surface expression of MHC II in PTEN (-/-) HCT 116 cells was markedly reduced by greater than 50% vs parental cells. PTEN (-/-) HCT 116 cells also expressed lower basal levels of MHC I, with similar attenuation in induction by IFNs. These reductions in surface expression correlated with lower MHC I/II mRNA levels. IHC studies in patient head & neck cancer biopsy samples demonstrated an inverse relationship between pAKT and MHC I expression. Additional studies were performed comparing PTEN knockout and the expression of STAT1, a downstream target of IFN signaling. We observed that PTEN/PIK3 signaling implicates STAT1, and that PTEN inhibition consistently modulated total and phosphorylated levels of STAT1 protein in PTEN (-/-) HCT 116 cells. Similar findings were also observed in melanoma cells. Conclusions: We aim to further characterize the link between oncogenic signaling and anti-tumor immunity. We demonstrate, using an in vitro model, that genetic and pharmacologic activation of the PI3K pathway modulates the immune response to IFNs and represses MHC induction and expression. Importantly, similar inverse patt